A liquid crystal display device used for a display screen of a mobile phone or a personal computer etc. is created by mounting a driving IC (liquid crystal driving IC) to a liquid crystal panel, which includes a liquid crystal layer sealed between a pair of substrates.
Incidentally, a TCP (Tape Carrier Package) method is used in general for mounting a driving IC with respect to a liquid crystal panel.
Further, in recent years, a COG (Chip On Glass) method of performing bear chip mounting of the driving IC to the liquid crystal panel is becoming more common in terms of cost reduction, improvement of reliability, realizing a thinner display etc. In this kind of method, the most common method is a method of carrying out face down bonding so as to connect an umbonal bump electrode, which is formed on the driving IC, to a pad (electric wiring) of the liquid crystal panel.
Note that, the pad is electrical wiring formed on a driving IC connection area of the display substrate (glass substrate) in the liquid crystal panel. Further, the pad includes signal wiring extended to the display section of the liquid crystal panel, and supplies a data signal and a scanning signal to the display section, an output bonding pad short-circuited to the signal wiring, supply wiring (input wiring) for supplying a signal and electric power (electric power source) to the driving IC, an input bonding pad short-circuited to the supply wiring, and an FPC input terminal for transmitting a signal and electric power supplied from an external circuit to the supply wiring. Note that, the FPC input terminal is short-circuited to the supply wiring and has a function of connecting to an FPC (Flexible Printed Circuit) for carrying out wire connection with an external circuit.
Further, as to the COG connection method, known is such as a method of forming a bump electrode of the driving IC by using a solder, and fusing the electrode into the pad of a liquid crystal panel so as to connect the electrode to the pad, or a method of forming a bump electrode by using a metal such as Au, and connecting the electrode to the pad by an anisotropic conductive adhesive. Here, the anisotropic conductive adhesive (AC adhesive) is an adhesive made of conductive paste or an insulative adhesive, which is mixed with conductive particles diffused therein. In the COG method, the one made of an insulative adhesive is adopted.
Particularly, in a connection method using the AC adhesive, the conduction between the driving IC and the liquid crystal panel is carried out by holding the conductive particles included in the adhesive between a bump electrode of the driving IC and a pad of the liquid crystal panel. In this manner, the connection pitch only depends on the size of the bump electrode. Further, the insulation property between each electrode can easily be improved by filling the insulative adhesive between the electrodes. In view of these advantages, the connection method using the AC adhesive is a mainstream in the COG method.
The following will explain connection operation (face down bonding) of a liquid crystal panel and a driving IC in case of using an anisotoropic conductive film (ACF) as the AC adhesive, with reference to FIGS. 2 through 4 according to the present invention.
For ease of explanation, materials having the equivalent functions as those of the present invention will be given the same reference symbols. Further, the ACF is an AC adhesive including conductive particles diffused in an adhesion film, which has the thickness of 15 μm to 45 μm.
FIG. 2 is a plan view schematically showing an arrangement of a liquid crystal display device. As shown in the figure, the liquid crystal display device includes a driving IC 4 and a liquid crystal panel 1.
Further, FIG. 3 is a cross-sectional view showing a connection portion for connecting the driving IC 4 and the liquid crystal panel 1. As shown in the figure, the driving IC is bonded with respect to a glass substrate 8 of the liquid crystal panel 1 through an ACF 10. Further, the conductive particles 11 diffused in an adhesion film of the ACF 10 are caught between a bump electrode 9 of the driving IC 4 and a pad 7 provided on the glass substrate 8. The bump electrode 9 and the pad 7 are electrically conducted by the conductive particles 11.
Here, the following will explain a method of bonding the driving IC 4 and the glass substrate 8 through the ACF 10 with reference to FIG. 4.
Firstly, the ACF 10 is placed in an area (mounting section 3) including the pad 7 of the glass substrate 8. Then, the pad 7 and the bump electrode 9 of the driving IC 4 are adjusted to be set in the same position. Thereafter, the driving IC 4 is subjected to thermo compression bonding with respect to the glass substrate 8 by using a compression bonding tool 6.
Through this process, the ACF is hardened, and the driving IC 4 and the glass substrate 8 are bonded. Here, as it is described in Japanese Laid-Open Patent Application Tokukaihei 10-206874/1998 (published on Aug. 7, 1998), the conductive particles 11 are deformed due to its elasticity by being caught between the pad 7 and the bump electrode 9. Further, since the deformed conductive particles 11 are held by the surrounding insulative adhesive, the pad 7 and the bump electrode 9 are electrically conducted to each other.
Note that, the AC adhesive is not always a film-shaped but can be in a state of paint, as with the one disclosed in Japanese Laid-Open Patent Application Tokukai 2000-150580/2000 (published on May 30, 2000). This publication also discloses a technique for placing a glass substrate on a circular arc-shaped compression bonding stage when the driving IC is subjected to compression bonding with respect to the glass substrate. This technique is used for the purpose of maintenance of flatness of the glass substrate at the time of thermo compression bonding, by relying on the shape of the compression bonding stage.
Incidentally, in the bonding method shown in FIG. 4, the thermo compression bonding is performed by pressing the compression bonding tool 6 to the driving IC 4. Therefore, the temperature of the driving IC 4 becomes higher than that of the glass substrate 8, and this brings about great thermal expansion of the driving IC 4. As a result, the AC adhesive is hardened and shrunk by having the glass substrate 8 and the driving IC 4, which are different to each other in their thermal expansion level.
Consequently, after the thermo compression bonding, difference occurs between the glass substrate 8 and the driving IC 4 in their shrinkage levels. This causes a bend at the bonding portion between the driving IC 4 and the glass substrate 8, as shown in FIG. 5.
The bend affects further in the display section 2 of the liquid crystal panel 1, and brings about a problem of unevenness of luminance of displayed images.
Further, in a larger liquid crystal display device, or a liquid crystal display device with higher definition, the device includes a plurality of driving ICs 4 bonded to the glass substrate 8. In such a liquid crystal display device, the described bend occurred at the bonding portion between the driving IC 4 and the glass substrate 8 may further cause waveform deformation of the glass substrate 8, as shown in FIG. 6.